Field of the Invention
The present invention relates to a technique of reading out a pixel signal in an image sensor.
Description of the Related Art
Conventionally, as a solid-state image sensor which converts entering light which forms an image of an object into an electric signal, a CMOS sensor obtained by integrating a photodiode and MOS transistor into one chip is used. A CMOS sensor is more advantageous in terms of, for example, low power consumption, low driving power, and speed-up than a CCD. In recent years, a method which exploits the CMOS sensor to increase the readout speed of a pixel signal using an analog/digital converter (ADC) provided for each column has been proposed.
For example, an image sensor having a structure in which two or more ADCs are provided to one pixel column of a pixel portion has been proposed (see Japanese Patent Laid-Open No. 2005-347932). This image sensor performs readout using a plurality of ADCs to improve the readout speed, while the conventional CMOS sensor reads out pixel signals of one pixel column using one ADC.
In another method, a successive approximation image sensor is available as an example in which high-speed ADCs are loaded (see U.S. Pat. No. 5,880,691). A successive approximation image sensor includes a voltage comparator, a digital memory, and a reference voltage generator which uses a digital/analog converter on each column. The voltage comparator receives a signal from a pixel at its one terminal, and a voltage from the reference voltage generator at its other terminal. The reference voltage generator sequentially changes the value based on the comparison result obtained by the comparator.
However, in the conventional method, the circuit scale of an ADC provided for each column increases. Even in the configurations described in Japanese Patent Laid-Open No. 2005-347932 and U.S. Pat. No. 5,880,691, the readout speed of a pixel signal can be increased, but the circuit of the ADC portion has a scale larger than the conventional CMOS sensor, so the chip size considerably increases.
On the other hand, assuming a camera, in most cases, an increase in readout speed of a pixel signal is required not when signal readout of all pixels in an image sensor widely used in still image capture is done but when pixel addition readout or thinning readout is performed for, for example, a moving image capture, photometry, or distance measurement operation.